Theme · Platform: caption buttons of their own height, titles that give way

Windows caption buttons take the bar's height less its hairline, a new
token, instead of their row's, so they keep their height beside caps
that wrap below them. A title bar's title is an Ellipsis: on Windows it
takes the free width before the buttons and gives way; a centered title
sits inset past the lights on both sides. The controls never shrink.
Tests hold a long Windows title's close button inside a 280px bar and
wrapped caps inside their row.

examples/gallery/pages/shell/chrome.rs

@@ -59,7 +59,7 @@ pub fn title_bar(cx: &App) -> impl IntoElement + use<> {
             })),
     )
     .child(code(
-        "TitleBar::new(id) with no style leaves room for the real macOS lights.",
+        "TitleBar::new(id) with no platform leaves room for the real macOS lights.",
         cx,
     ))
 }